U.S. Pat. No. 4,415,345 discloses a process for rejecting the nitrogen from the methane in a double rectification column operating at cryogenic temperatures. A double rectification column comprises a higher pressure rectification column, a lower pressure rectification column, and a condenser-reboiler placing the top of the higher pressure rectification column in indirect heat exchange relationship with a region, usually the bottom, of the lower pressure rectification column. In the process according to U.S. Pat. No. 4,415,345 a stream of a mixture of nitrogen and methane at elevated pressure is cooled to a temperature suitable for its separation by rectification. The stream is at least partially liquefied by passage through a condenser-reboiler associated with the bottom of the lower pressure rectification column. A part of the bottom liquid fraction separated in this column is therefore reboiled to provide an upward flow of vapour through the column. Remaining liquid is employed as a feed to the lower pressure rectification column in which a relatively pure liquid methane product is separated as the bottom fraction. A stream of the resulting liquid methane is withdrawn from the lower pressure column and is raised in pressure by a pump. It is warmed by heat exchange to approximately ambient temperature and is thus vaporised. In order to enhance the liquid nitrogen reflux available to the double rectification column the stream of nitrogen is withdrawn from the top of the lower pressure rectification column, is compressed in a compressor to the operating pressure of the higher pressure rectification column and is combined with a nitrogen stream flowing from the top of the higher pressure column to the condenser-reboiler thermally linking the lower pressure rectification column thereto. As a result, the compressed nitrogen is condensed. A part of the condensate is returned to the lower pressure rectification column and another part to the higher pressure rectification column. The flow of fluid from the lower pressure rectification to the higher pressure rectification column via the compressor acts as a heat pump. A heat exchanger is incorporated in the heat pump so as to remove heat of compression. The heat exchanger is separate from those in which the pumped product methane stream is warmed.
At certain nitrogen concentrations, typically in the range of 15 to 30% by volume in the feed gas, a pinch occurs in the lower pressure rectification column. We have discovered that the position of this pinch point depends on the nitrogen concentration. As the nitrogen concentration approaches 30% so the higher becomes the position of the pinch. This gives rise to difficulties in the separation process and can lead to either a reduction in the methane recovery or an increase in the specific power consumption.